By means of a belt height adjustment means the run of a safety belt may be optimally adapted to the stature of a vehicle occupant in each case. Usually a deflector fitting for the belt system is adjusted in height involving the use of a detent mechanism which engages the desired detent opening in a rail. If, however, the position of the sled is not so exactly set that the detent mechanism can snap into a detent opening but remains between two detent openings, then the tension force occurring in the safety belt will result in a displacement of the sled in an accident so that the restraining action desired of the safety belt is not obtained as early on as desired. This problem is made even more acute, if the safety belt system is provided with a belt pretensioner which is to eliminate the belt slack from the belt system in case of an accident. The forces thereby additionally exerted on the belt height adjuster may even result in the detent mechanism fails to snap into the next detent opening after the displacement of the sled and slips thereover, as a result of which the effect normally to be produced by the belt pretensioner is impaired. Therefore, detent mechanisms have been proposed which intend to ensure engagement of the corresponding next detent opening when the sled is caused to move downwards relative to the vehicle with a high acceleration rate. Such known detent mechanisms are, however, not sufficiently reliable in operation so that, even if they are used, the action of the belt pretensioner may still be impaired.